Liquid-crystal display devices are conventionally constituted of a pair of transparent electrode substrates provided opposite to each other at a certain distance, a liquid-crystal alignment film of polyimide or the like coming into contact on each side with liquid crystal, and a liquid crystal hermetically sealed between the electrode substrates. One liquid-crystal display device has a large number of pixels, and images are displayed by utilizing changes in the direction of alignment of liquid-crystal molecules at the pixel portions, the changes being caused upon application of a voltage to the liquid-crystal layer through transparent electrodes. In recent years, liquid-crystal display devices have become available that incorporate switching devices such as thin-film transistors (TFT) on the electrode substrates at the pixel portions. These are known as active matrix type liquid-crystal display devices. The active matrix type liquid-crystal display devices commonly employ a twisted nematic (TN) type liquid-crystal display system in which the alignment direction of liquid-crystal molecules is twisted by about 90 degrees from one electrode substrate toward the other electrode substrate at the time the voltage is not applied across the electrodes. In the liquid-crystal display devices of the TN type liquid-crystal display system, the application of a voltage across the electrodes causes liquid-crystal molecules to incline to make it possible to perform gradational display. However, since this inclination of liquid-crystal molecules has a directionality, a visual angle dependence comes into question such that display colors or contrast ratios vary depending on the direction in which liquid-crystal display devices are viewed.
Some methods for improving this visual angle dependence to broaden the visual angle are disclosed as a pixel division method in which display electrodes constituting one pixel are divided and the voltage applied thereto is changed for each electrode (Japanese Patent Application Laid-open No. 2-12) and an alignment division method in which the insides of pixels are divided to change pretilt angles (Japanese Patent Application Laid-open No. 62-159119) or to change the alignment direction of liquid-crystal molecules (Japanese Patent Application Laid-open No. 63-106624), for each region.
In liquid-crystal display devices, the liquid-crystal alignment film formed on an electrode substrate on the side coming into contact with the liquid crystal has the function to align the liquid-crystal molecules in a certain direction on the film surface. For this purpose, treatment called rubbing is prevalent. Rubbing is a process in which a high-molecular film of polyimide or the like formed on the substrate is rubbed with a cloth covered with a large number of fibers like velvet to thereby furnish the film with the ability to cause liquid-crystal alignment, and is a simple and inexpensive process.
As liquid-crystal alignment control processes other than the rubbing, proposed are an oblique deposition process making use of an obliquely deposited film of SiO or the like (Japanese Patent Application Laid-open No. 56-66826, etc.), a photolithographic process in which irregularities are formed in gratings on the surface of an alignment film by a process such as photolithography (Japanese Patent Application Laid-open No. 60-60624, etc.), an LB film process in which high-molecular chains are aligned in the draw-up direction when built up on the substrate (Japanese Patent Application Laid-open No. 62-195622, etc.), an ion irradiation process in which ions are obliquely shed (Japanese Patent Application Laid-open No. 3-83017, etc.), a high-velocity fluid jet process in which a fluid is obliquely jetted at a high velocity (Japanese Patent Application Laid-open No. 63-96631), an ice blasting process in which ice pieces are obliquely blown (Japanese Patent Application Laid-open No. 63-96630), an excimer laser process in which a high-molecular surface is irradiated with excimer laser light or the like to form a periodic stripe pattern (Japanese Patent Application Laid-open No. 2-196219, etc.), an electron-ray scanning process in which fine irregularities are formed by scanning the surface of a thermoplastic material with electron rays (Japanese Patent Application Laid-open No. 4-97130, etc.), a centrifugal process in which a centrifugal force is caused to act on a coated alignment film-forming solution, to align high-molecular chains (Japanese Patent Application Laid-open No. 63-213819), a stamp process in which a base material having already been aligned is contact-bonded to transfer the ability to cause alignment (Japanese Patent Application Laid-open No. 6-43457, etc.), Y. Toko et al., a random alignment process in which a chiral agent is added to twist the structure (J. Appl. Phys., 74 (3), p.2071, 1993), M. Schadt et al., a photo-dimerization process that utilizes two-plus-two addition cyclization reaction of polyvinyl cinnamate (Jpn. J. Appl. Phys., 31 Part 1, No. 7, p.2155, 1992), Hasegawa et al., a photo-decomposition process in which a polyimide film is photo-decomposed with polarized ultraviolet light (Liquid-crystal Forum Draft Collections, p.232, Article No. 2G604, 1994) and so forth, none of which, however, have been put into industrial application.
Meanwhile, W. M. Gibbons et al. report that the alignment direction of liquid-crystal molecules on the surface of an alignment film can be changed in the direction perpendicular to the direction of an electric field of irradiation polarized light when a polyimide liquid-crystal alignment film is previously doped with a diazodiamine coloring matter to prepare a cell in which liquid crystal has been aligned in a certain direction by rubbing and the resultant cell is irradiated with polarized laser light (Nature, 351, p.49, 1991). Ichimura, who is one of the present inventors, also reports that liquid-crystal molecules can be switched between vertical alignment and parallel alignment simply by irradiation with light and also the direction of parallel alignment can be changed by irradiation with polarized light, when the surface of a liquid-crystal display device substrate is chemically modified with photochromic molecules (Applied Physics, 62 (10), p.998, 1993, etc.). All of this research is pioneer research for next-generation photo-driven type liquid-crystal display devices which are different from electric-field driven type liquid-crystal display devices like the liquid-crystal display devices presently put into practical use.
In the case when the rubbing is used to align liquid-crystal molecules in a certain direction on the film surface, problems may arise such that fibers may come off a rubbing cloth at the time of rubbing, dust tends to adhere because of static electricity generated at the time of rubbing, the substrate may be scratched because fibers and dust are rubbed against it, cell gaps may become poor when the panel is assembled in the state where fibers and dust have adhered and, in order to prevent this, the step of cleaning the substrate after rubbing is required, and some materials for alignment films cause deterioration of alignment performance as a result of washing. In the case of active matrix type liquid-crystal display devices, the static electricity may cause breakage of switching devices. Moreover, it is difficult for substrates with irregularities or substrates with a large area to be uniformly rubbed.
Among the liquid-crystal alignment control processes other than the rubbing, the oblique deposition process, ion irradiation process and electron-ray scanning process require vacuum equipment and are not practical for processing large-area substrates. The centrifugal process requires large-scale equipment for carrying out one-direction alignment on large-area substrates, and is not practical.
The high-velocity fluid jet process and ice blasting process can attain uniform alignment only with difficulty when large-area substrates are processed.
The stamp process is not suited for mass production because the substrate used in the mold can be repeatedly utilized only a small number of times.
The random alignment process can form fine domains and hence can provide broad visual angles, but has the problem of a contrast decrease caused by leakage of light from domain boundaries.
The photo-decomposition process is accompanied by a decrease in molecular weight of the polyimide used as an alignment film material, and hence has a possibility of a decrease in film strength or a decrease in resistance to liquid crystals.
The photolithographic process requires complicated steps and is not practical.
The LB process takes a long time for build-up and is not suited for mass production.
The excimer laser process requires expensive equipment and also can not be applied in thin films like those conventionally used in liquid-crystal alignment films. Moreover, it requires scanning or the like in order to process large-area substrates and takes a long time.
In the processes where irregularities are formed in gratings, as in the photo-dimerization process, electron-ray scanning process the photolithographic process and the excimer laser process, it is difficult to generate pretilt angles.
In addition, in order to produce liquid-crystal display devices having a wide visual angle, the processes other than the random alignment process require pixel division or alignment division. The pixel division, however, is not very effective for the improvement in visual angle dependence. With regard to the alignment division, the liquid-crystal alignment control processes other than the rubbing have difficulty in achieving it, or require complicated steps.
Meanwhile, in an attempted coloring matter doping process as proposed by W. M. Gibbons et al., in which a polyimide liquid-crystal alignment film is doped with a diazodiamine coloring matter, utilized to form liquid-crystal alignment films of electric-field driven type liquid-crystal display devices, there is a possibility that the coloring matter, having a low molecular weight, may diffuse into the liquid-crystal layer in the course of time, resulting in a loss of the ability to cause liquid-crystal alignment or damage of the display performance required for liquid-crystal display devices. Thus, if the liquid-crystal alignment films designed for photo-driven type liquid-crystal display devices are attempted being utilized as liquid-crystal alignment films of electric-field driven type liquid-crystal display devices, the alignment films may have insufficient stability in the ability to cause liquid-crystal alignment.